The first thing you have to understand about “Rock of Ages” is that it’s really a remarkably dirty, old-school musical.

The sex is both raunchy and mostly clothed, the strippers are oddly dignified, love is expressed in ballad format, music has the power to soothe even the most savage beast, and most of the cast has that clueless, deranged optimism that makes teens think they can conquer the big city with nothing more than a suitcase full of records.

The resulting movie has a serious case of split-personality disorder, too naughty for the usual musical crowd and too toothless for audiences looking for some real rock and roll.

The plot is a fondue of cliches including a small town girl in the big city, a struggling rocker makes it big, a megastar begins to question the meaning in his life, a girl sees through the aforementioned megastar’s jaded exterior, the unscrupulous agent and the terrible romantic misunderstanding that would have been solved in five minutes if anyone actually used their heads. There’s also a group of boy band rappers (I was slightly confused on how they got there) and enough suggested sex (and talk about sex) that I wouldn’t let children or grannies anywhere near this thing.

The biggest warning, and I never thought I’d actually need to be writing these words in this particular order, is the moment of sudden and completely random homosexuality that happens two-thirds the way through the film. I’m pretty sure the two men involved only express their love in ‘80s ballad format, but as I was hiding at the time I can’t really be sure. There’s no hint of it before and no mention of it again, but during the length of the song they’re obvious about it.

The music, which is the entire reason the original stage show got put together in the first place (yes, kids, this is on Broadway), isn’t even all that great. Tom Cruise is a surprisingly good singer, Julianne Hough is unsurprisingly the same (at least with the pop-style stuff), and Mary J. Blige is a gift the movie doesn’t deserve. All the songs, however, essentially feel like covers, missing any of the grit and screaming guitars that made the originals so much fun.

Most of the fun in “Rock of Ages” comes from a handful of excellently tossed-off jokes by Tom Cruise (astonishingly), Paul Giamatti, Alec Baldwin, and Russell Brand, along with the entire five-minute scene starring T.J. Miller as a nameless Rolling Stone receptionist.

True, hardly any of it would be printable in the newspaper (Miller’s scene, and a riff on the habits of a Firebird by Cruise are among the few exceptions), but they add some much-needed entertainment value. They don’t give Catherine Zeta-Jones any good lines, but her sheer enthusiasm almost makes up for it.

Tom Cruise also adds a few moments of what I’m pretty sure is genuine depth to the proceedings, alternating between what I imagine is the world’s greatest Axl Rose impression and giving us genuine glimpses of a man who has completely lost himself and knows it. When he says “I’m a slave to rock and roll,” he actually makes the words sound a little believable.

In the end, though, you’ll probably be more satisfied with loading an ultimate ‘80s mix onto an mp3 player. The resulting experience will certainly rock a lot harder than the movie does.